Passive Data Transmission

ABSTRACT

Data is communicated from an implanted section to an external location. The implanted section includes a transducer such as a piezoelectric element with an ultrasound reflecting surface that moves in response to an applied driving signal. A sensor generates an output signal that depends on a sensed parameter, and a control circuit drives the transducer based on the sensor&#39;s output. The transducer&#39;s response to the driving signal is repeatable such that the value of the output signal can be determined by measuring the variations in the velocity of the surface using externally applied Doppler ultrasound, and computing the value of the sensor&#39;s output from the measured variations in velocity.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application61/864,216, filed Aug. 9, 2013, which is incorporated herein byreference in its entirety.

BACKGROUND

This invention relates to communication between two or more devicespositioned at a distance from each other, where power consumption is alimiting factor.

One example where this can occur would be transmitting data gathered bya battery-powered sensor that is implanted within the human body to theoutside world over a long period of time. In this example, especiallywhen continuous monitoring and transmission is being made ofphysiological, chemical or physical parameters, the battery power maynot be sufficient for the transmission to continue for a sufficientlylong time when data transmission is implemented using conventionaltechniques. As a result, the battery may require replacement by aninvasive procedure.

Another example is when data or instructions/commands are to betransmitted from one point within a body to a second point in the samebody. For example, it may be desirable to have a sensor that measurespulmonary vein blood pressure transmitted pressure data to a cardiacpacemaker in order to optimize the pacemaker's performance. Once again,battery power may not be sufficient and the battery may requirereplacement by an invasive procedure.

Examples of using ultrasound to detect an implanted device are describedin Detection of Deeply Implanted Impedance-Switching Devices UsingUltrasound Doppler by J. M Mari et al, IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency Control, vol. 60, no. 6, June 2013, at1074-1083. This article is incorporated herein by reference in itsentirety.

Examples of using ultrasound to communicate with an implanted device aredescribed in Deeply Implanted Medical Device Based on a Novel UltrasonicTelemetry Technology by Michela Peisino, Thèse no 5730, ÉcolePolytechnique Fédérale de Lausanne, May 17, 2013. This paper isincorporated herein by reference in its entirety.

Examples of using ultrasound for energy transfer and communication withimplanted devices are described in Ultrasound for Wireless EnergyTransfer and Communication for Implanted Medical Devices by F. Mazzilliand C. Dehollain, ESSCIRC 2010, Workshop, Seville, Sep. 17, 2010. Thispaper is incorporated herein by reference in its entirety.

But the impedance-switching based technology described in thesereferences is inadequate, and improved approaches for communicating withimplanted devices are needed.

SUMMARY OF THE INVENTION

One aspect of the invention is directed to an apparatus that includes animplanted section and an external section. The implanted sectionincludes (a) a transducer having an ultrasound reflecting surface thatmoves in response to an applied driving signal, wherein variations inthe applied driving signal causes corresponding variations in thevelocity of the surface, (b) a sensor that generates an output signalthat depends on a sensed parameter, and (c) a first control circuit thatgenerates the driving signal that is applied to the transducer based onthe output signal generated by the sensor, such that variations in thevalue of the output signal result in corresponding variations in thedriving signal. The transducer's response to an applied driving signaland the first control circuit's response to the output signal arerepeatable such that the value of the output signal can be determined bymeasuring the variations in the velocity of the surface using externallyapplied Doppler ultrasound. The external section includes (1) anultrasound transmitter configured to direct ultrasound energy at acarrier frequency onto the transducer, (2) an ultrasound receiverconfigured to detect ultrasound reflections from the surface that havebeen shifted in frequency by the moving surface, (3) a Doppler processorconfigured to determine the velocity of the surface based on thedetected reflections, and (4) a second control circuit configured to mapthe determined velocity onto a value of the sensed parameter.

In some embodiments the transducer is a piezoelectric element. In someembodiments, the implanted section further includes a biocompatiblehousing, and the transducer, the sensor, and the first control circuitare all housed in the housing. In some embodiments, the first controlcircuit includes a processor and a driver circuit. In some embodiments,the driving signal is selected to cause FM modulation of the carrierwhen the surface moves. In some embodiments, the driving signal is atriangular waveform. In some embodiments, the driving signal has afrequency between 50 and 1000 Hz. In some embodiments, the carrierfrequency is between 1 and 20 MHz. In some embodiments, the movement ofthe surface is a vibration.

Another aspect of the invention is directed to a sensing apparatus thatincludes a transducer having an ultrasound reflecting surface that movesin response to an applied driving signal, wherein variations in theapplied driving signal causes corresponding variations in the velocityof the surface. This apparatus further includes a sensor that generatesan output signal that depends on a sensed parameter, and a controlcircuit that generates the driving signal that is applied to thetransducer based on the output signal generated by the sensor, such thatvariations in the value of the output signal result in correspondingvariations in the driving signal. The transducer's response to anapplied driving signal and the control circuit's response to the outputsignal are repeatable such that the value of the output signal can bedetermined by measuring the variations in the velocity of the surfaceusing externally applied Doppler ultrasound.

In some embodiments, the transducer is a piezoelectric element. In someembodiments, the sensing apparatus further includes a biocompatiblehousing, wherein the transducer, the sensor, and the control circuit areall housed in the housing. In some embodiments, the sensing apparatusfurther includes a battery that provides power to the sensor and thecontrol circuit, and the battery is housed in the housing. In someembodiments, the control circuit includes a processor and a drivercircuit. In some embodiments, the driving signal has a frequency between50 and 1000 Hz. In some embodiments, the movement of the surface is avibration.

Another aspect of the invention is directed to a method forcommunicating with an implanted sensor. This method includes the stepsof (1) obtaining an output signal from a sensor, wherein the outputsignal depends on a sensed parameter; (2) generating, based on theoutput signal obtained from the sensor, a driving signal for driving atransducer, such that variations in the value of the output signalresult in corresponding variations in the driving signal, wherein thedriving signal is configured to cause an ultrasound reflecting surfaceof the transducer to move, and wherein variations in the applied drivingsignal causes corresponding variations in the velocity of the surface;(3) applying the driving signal to the transducer, wherein thetransducer's response to the applied driving signal and the generationof the driving signal based on the output signal obtained from thesensor are repeatable such that the value of the output signal can bedetermined by measuring the variations in the velocity of the surfaceusing externally applied Doppler ultrasound; (4) directing ultrasoundenergy at a carrier frequency onto the transducer; (5) detectingultrasound reflections from the surface that have been shifted infrequency by the moving surface; (6) using Doppler processing todetermine the velocity of the surface based on the detected reflections;and (7) mapping the determined velocity onto a value of the sensedparameter.

In some embodiments, the driving signal is selected to cause FMmodulation of the carrier when the surface moves. In some embodiments,the driving signal is a triangular waveform. In some embodiments, thedriving signal has a frequency between 50 and 1000 Hz. In someembodiments, the carrier frequency is between 1 and 20 MHz. In someembodiments, the movement of the surface is a vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system for communicating with animplanted device.

FIG. 2 is a block diagram of communication with a plurality of implanteddevices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the preferred embodiment uses an implanted devicethat changes the velocity of an implanted transducer such as apiezoelectric element. The changes in velocity are detected usingDoppler ultrasound. This embodiment includes two main sections: animplanted section 200 and an external section 100.

Instead of relying on internally-supplied battery power to transmit asignal from the implanted section to the external section, the preferredembodiments rely on a carrier signal that originates outside the body.The implanted section 200 changes the velocity of an ultrasoundreflecting surface of a transducer (e.g., a Piezoelectric ReflectingElement 202, hereinafter “PRE”). When the external section 100 directsan ultrasound beam onto the PRE 202 in the implanted section 200, thechanges in velocity of the surface of the PRE 202 causes a frequencyshift, and that frequency shift is detected externally. This approach issuperior to the prior art mentioned above, which merely changes theimpedance of the implanted transducer.

This approach also saves power because modulation of an externallyapplied signal can be accomplished using less power than it would taketo have the implanted section transmit the signal by itself. Thisreduction in power consumption extends the life of the battery in theimplanted section with respect to conventional data transmissionmodalities.

The Passive Data Transmission System includes one or more implantedsensors designed to measure or monitor physiological, chemical orphysical parameters, etc. from the location where sensing can be carriedout to either (a) a location outside the patient's body or (b) toanother devices that is implanted in a different location in the body.

FIG. 1 depicts a preferred embodiment that includes two main sections:an implanted section 200 and an external section 100. It relies on atransducer (e.g., PRE 202) to modulate ultrasound beams 120 that aregenerated by the external section and aimed towards the implantedsection. In alternative embodiments, other transducers besidespiezoelectric reflecting elements may be used to produce the vibrations(e.g., a miniature electromagnetic speaker).

The external section 100 is also referred to herein as theEnergizing/Transmitting System (TS) and it preferably includes anultrasound Doppler transmitter 104 and ultrasound Doppler receiver 105;a processor 114 and 112 that controls the system and deciphers the codedsignals; a power supply 113; and a housing 108. The processor preferablyincludes two processing steps: the first step is Doppler shiftprocessing 114, where the velocity of the surface of the PRE isdetermined. The second is the decipher processing step 112, where thevelocities determined in the first step are mapped onto a value of thesensed parameter. Although depicted as two discrete blocks in FIG. 1,these two processing steps may optionally be implemented by a singleprocessor (not shown). Optionally, a display 122 may be used to displaythe result of the mapping. Alternatively, the result of the mapping maybe transmitted to an external device.

The implanted section 200 is also referred to herein as the sensingsystem (SS) and it preferably includes a sensor or sensors 201 withassociated amplifiers, etc.; a processor 215 configured to control andactivate the sensors 201 and to interpret return signals from thesensors; a driver circuit 217 that translates the output of theprocessor 215 into an electric signal using a modulation scheme(including but not limited to FM, AM or another modulation scheme; a PRE202 such as a small disc that is activated (i.e. made to move e.g., tovibrate) by the modulated electric signals. All of these components arepowered by a suitable power source (e.g., battery 207) and arepreferably housed in a biocompatible enclosure 208.

The sensor 201 of the SS 200 senses the relevant biological, chemical,or physical parameter. Any of a wide variety of conventional sensors canbe used for this purpose, depending on the anatomical function that isbeing monitored. Preferably, the sensor 201 transduces the parameterthat is being sensed into an electric signal. When necessary, theensuing electric signals are amplified, filtered, shaped, etc., and theresulting signal is provided to a processor 215 via a suitableinterface.

The processor 215 has the ability to drive the PRE 202 by sendingappropriate signals (e.g., pulses) to a driver circuit 217, such thatwhen the driver circuit 217 drives the PRE 202 in response to thesignals from processor 215, the PRE 202 responds mechanically to thedriving signals by moving in a repeatable manner (e.g., by vibrating).Collectively, the processor 215 and the driver circuit 217 make up acontrol circuit.

The processor 215 encodes desired output data into the signals thatcontrol the PRE 202 (where the output data represents information thatwas obtained from the sensor). The PRE 202 mechanically responds tothose signals by moving in a repeatable manner, and the mechanicalactivity of the PRE 202 is then detected by the external section 100.The system relies on this repeatability to deliver information from theimplanted section to the outside world. A preferred way to detect themechanical activity of the PRE is by using Doppler ultrasound.

Doppler ultrasound is beneficial because it detects velocities. So inorder to convey information from the SS 200 to the TS 100, the SS 200controls the velocity of the PRE 202 contained within the SS 200. To dothis, the processor 215 encodes the data that is wants to send outsidethe body onto a signal that causes the PRE 202 to move (e.g., vibrate)in a repeatable manner. For example, the shape, duration, time line,frequency etc. of a vibration can be used to convey the information thatwas obtained by the sensor. The vibration of the PRE 202 will thenconvey all the required information. (Note that the information detectedby the Doppler system is preferably contained in the vibrating reflectorvelocity or the velocity profile with time.)

In one example, the information may be coded in the frequencies of atriangular waveform, in which case the velocity is a square wave ofcorresponding frequencies. Assuming that the frequency of the signalthat is applied to the PRE 202 is kept constant, increasing theamplitude of a triangular waveform that drives the PRE will increase theamplitude of the mechanical vibrations. This will increase the velocityof the PRE 202 as it passes through the midpoint of the vibration. Thisincreased velocity can then be picked up by the TS 100.

In another example, the information can be conveyed by generating aseries of pulses, and encoding the information onto the pulses. Forexample, a wide pulse can represent a 1 and a narrow pulse can representa 0. Alternatively, a pulse can represent a 1 and the absence of a pulsecan represent a 0. A wide variety of alternative modulation schemes canbe readily envisioned.

The motion of the ultrasound reflecting surface of the PRE 202 (e.g.,the vibrations) can then be detected by the TS 100, using Dopplerultrasound in a conventional manner. For example, in the embodimentdepicted in FIG. 1, the TS 100 is positioned on the body surface suchthat it can easily be provided with the necessary power. The TS 100 inthis embodiment is an ultrasound Doppler system, preferably withoutimaging (e.g., a 2 MHz pulsed Doppler system). In alternativeembodiments, other frequencies may be used, e.g., between 1 and 20 MHz.

An ultrasound transmitter 104 emits a wave/beam 120, and that beam isdirected at the selected SS 200. Ultrasound energy 220 is reflected backfrom the PRE 202 in the SS 200. As explained above, the velocity of themoving surface of the PRE 202 will depend on the signals that areapplied to the PRE 202 within the SS 200. As a result, the ultrasoundenergy 220 reflected back from the PRE 202 will be shifted in frequency(i.e., Doppler shifted) by the motion of the PRE 202. (Note that sincethe PRE 202 moves in response to the encoded data, that movement can bedetected, and the data can be extracted from the detected movements.)The Doppler shifts are picked up by the ultrasound receiver 105 and theDoppler shifts are isolated and processed in block 114 to determine thevelocity of the surface of the PRE.

Decipher processing 112 is then implemented, where the determinedvelocities are mapped onto a driving signal waveform that is known toproduce the velocities that are measured. Then, the driving waveform canbe mapped onto the value of the sensed parameter based on knowledge ofwhich driving waveforms are generated by the driver circuit 217 inresponse to a given output levels from the sensor 201. In other words,because the TS 100 is aware of the transfer function of the sensor 201,the processor 215, and the driver circuit 217, and can compute the valueof the sensed parameter from the measured velocities in the Decipherprocessing block 112. Decipher processing 112 is preferably implementedin a control circuit such as a processor, e.g., using a lookup table tomap the determined velocities onto values of the sensed parameters.

Optionally, the resulting data can then be displayed or transmitted.

The coded vibration velocities are thereby translated so as to deliverthe Sensor-derived information (i.e., the value of the sensed parameter)to the outside world. The information thus obtained can be displayed,transmitted to the physician, a medical unit, another implanted devicethat uses it to adjust its function, etc. in any conventional manner.

As an example, the system depicted in FIG. 1 may be used to transmitdata from a blood pressure sensor implanted in an artery to a Receiverpositioned on the body surface. In this case, the sensor 201 would be apressure transducer that transduces the blood pressure into an electricvoltage that depends on the pressure. The sensor may be configured todetect the systolic and/or diastolic pressures (e.g., refreshed every3-10 sec). Alternatively, the whole pulse wave contour can be sensed,e.g., with pressure values obtained at a rate of 20-100 Hz. In thisexample, the output of the sensor 201 is an analog signal. The analogsignal is then digitized and fed to the processor 215.

The processor 215 transforms the digitized voltage into a coded voltagewaveform such that its frequency or other characteristics of thewaveform varies with time in correspondence to the pressure values thatwere sensed by the sensor 201. This waveform is applied to the drivercircuit 217, which generates a driving signal that drives the PRE 202and causes the PRE to vibrate accordingly. The driving signal preferablyhas a frequency content that is within the range of the frequencyresponse of the PRE 202.

The sensed pressure is preferably coded using a code that is impartedonto the velocity of movement of the PRE 202, rather than its amplitude,etc. For example, if the driving signal has a triangular waveform, thevelocity that is detected using Doppler will be a square wave of thesame periodicity. In such a case the coding can be in the duration ofindividual triangular wave cycle times. A repeatable mapping is used(e.g., 100 microsecond of pulse width of per millivolt of signal fromthe sensor). Different voltages from the sensor will therefore beregistered as square waves of different duration. This is a form offrequency modulation.

Alternatively, the slopes of the triangular waves can be changes, whichwill increase the velocity of the PRE, and will therefore be registeredas square waves of different amplitude, i.e. an amplitude modulationcode. The information can also be coded in the duration or shape of aburst of waves or vibration.

The Doppler registered velocity is obtained by a TS 100 that ispreferably positioned at the surface of the body using an appropriateultrasound impedance matching gel between the TS 100 and the body. Astandard Doppler ultrasound system, preferably pulse Doppler, includesthe transmitter 104. The ultrasound beam, for example 2 MHz, is aimed atthe PRE 202, and the PRE reflects the beam.

The vibration or movement of the ultrasound reflecting surface of thePRE 202 will cause a Doppler shift in the 2 MHz waveform that isreflected back to the receiver 105 (which is also included in thestandard Doppler ultrasound system). Using conventional Dopplertechnology by means of an analogue mixer or digital arithmetic tools theshift in frequency from the original wave is extracted from the combinedwaveform. These waveforms contain the encoded information.

The SS 200 can function independently as explained above under controlof the Controller 203. Alternatively, it can be activated and controlledby signals from the TS 100 where Controller 106 determines the activityprotocol. The transmission of the commands etc. from the controller 106in the TS 100 to the controller 203 in the SS 200 may be implementedusing any conventional communication modality by employing a controltransmitter 131 in the TS 100 and a corresponding receiver 232 in the SS200. For example, ultrasound, magnetic field or RF (e.g., Bluetooth) maybe transmitted by Control Transmitter 131 in the TS 100 and received byReceiver 232 in the SS 200 to communicate.

In some embodiments, the PRE 202 can be integrated into the receiver232. The SS 200 can also be activated by means of a remote control. Whenthe relevant information can be derived in a relatively short time andis required only periodically, for example every 5 min, the reflectorcan be activated periodically at predetermined times and the transmittercan be synchronized to this periodicity.

In applications where the sensor reporting requires only a relativelysmall duty cycle, the TS transmission to PRE 202 can be utilized togenerate electric currents that charge the SS battery 207 (and/or acapacitor, (not shown). The delivery of power in this mode is describedin the Mazzilli reference identified above, which is incorporated hereinby reference. Alternatively, other piezoelectric elements that do notact as reflectors can be used for a similar purpose (i.e., harvestingenergy).

The PRE 202 preferably has a very high impedance and is activated by lowvoltages so as to consume an extremely low current such that it will notdrain the SS battery. Furthermore, as the piezo impedance is an inversefunction of frequency, low frequency waveforms for activating thevibration of the PRE 202 may be preferable. The PRE 202 may optionallybe coated by a highly reflective material (having a sound velocity verydifferent from that of tissues). Preferably, no air (including lungtissue that contains air) should intervene between the PRE 202 and thetissues separating it from the TS 100, since air will dampen thevibration signal and strongly attenuate the ultrasound beam.

Note that in the embodiments described herein, the PRE 202 does notgenerate a propagating wave itself Instead, it modulates an incomingsignal. Advantageously, very low power is required because the powerconsumption of the implanted SS 200 will relate only to the sensingitself and to the activation of the PRE 202 (i.e., causing the PRE tomove so that the motion will shift the frequency with respect to theincoming ultrasound beam), and not to actually transmit the data to theoutside world.

Note that the SS 200 is preferably implanted at an orientation such thatthe

PRE 202 faces the ultrasound beam generated by the TS 100.

Note that a large number of SS 200 implants may be distributed in asingle body, as depicted in FIG. 2 (e.g., F1 through F4). The TS 100directs ultrasound energy 120 at a carrier frequency onto thepiezoelectric elements contained in each SS 200, so that changes of thevelocity of a surface of the piezoelectric element shifts the frequencywith respect to the carrier. The TS 100 then detects the frequencyshifted ultrasound reflections 220 from the SS 200. Note that aplurality of SS devices 200 can even be read simultaneously by a singleTS 100 provided that in each the sensed information is coded withdifferent frequencies.

Note that Structures such as bone or air filled lung can interfere withultrasound imaging due to scattering. But Doppler measurements arepossible in spite of the scattering and attenuation, as explained in Y.Palti et al. Pulmonary Doppler Signals: a Potentially New DiagnosticTool Eur. J. Echocardiography 12; 25-31 (2011) and Y. Palti et al.Footprints of cardiac mechanical activity as expressed in lung Dopplersignals, Echocardiography, in press (2014).

Note that while the preferred embodiment described above utilizesultrasound beams, other waves, such as RF, can replace the ultrasound inless preferred embodiments. However, those other waves are lesspreferable than ultrasound because the attenuation for RF is 60-90 dB(at 2.45 GHz) and the attenuation for magnetic fields it is 50 dB (at 1MHz), as compared to the ultrasound which has a relatively lowattenuation. More specifically, the attenuation of ultrasound in aliving body over a typical distance of 10-20 cm, should be only 8-16 dB(at 1 MHz). In addition, Doppler processing is common for ultrasound.

While the present invention has been disclosed with reference to certainembodiments, numerous modifications, alterations, and changes to thedescribed embodiments are possible without departing from the sphere andscope of the present invention, as defined in the appended claims.Accordingly, it is intended that the present invention not be limited tothe described embodiments, but that it has the full scope defined by thelanguage of the following claims, and equivalents thereof.

What is claimed is:
 1. An apparatus comprising: an implanted sectionthat includes (a) a transducer having an ultrasound reflecting surfacethat moves in response to an applied driving signal, wherein variationsin the applied driving signal causes corresponding variations in thevelocity of the surface, (b) a sensor that generates an output signalthat depends on a sensed parameter, and (c) a first control circuit thatgenerates the driving signal that is applied to the transducer based onthe output signal generated by the sensor, such that variations in thevalue of the output signal result in corresponding variations in thedriving signal, wherein the transducer's response to an applied drivingsignal and the first control circuit's response to the output signal arerepeatable such that the value of the output signal can be determined bymeasuring the variations in the velocity of the surface using externallyapplied Doppler ultrasound; and an external section that includes (1) anultrasound transmitter configured to direct ultrasound energy at acarrier frequency onto the transducer, (2) an ultrasound receiverconfigured to detect ultrasound reflections from the surface that havebeen shifted in frequency by the moving surface, (3) a Doppler processorconfigured to determine the velocity of the surface based on thedetected reflections, and (4) a second control circuit configured to mapthe determined velocity onto a value of the sensed parameter.
 2. Theapparatus of claim 1, wherein the transducer comprises a piezoelectricelement.
 3. The apparatus of claim 1, wherein the implanted sectionfurther comprises a biocompatible housing, wherein the transducer, thesensor, and the first control circuit are all housed in the housing. 4.The apparatus of claim 1, wherein the first control circuit comprises aprocessor and a driver circuit.
 5. The apparatus of claim 1, wherein thedriving signal is selected to cause FM modulation of the carrier whenthe surface moves.
 6. The apparatus of claim 1, wherein the drivingsignal comprises a triangular waveform.
 7. The apparatus of claim 1,wherein the driving signal has a frequency between 50 and 1000 Hz. 8.The apparatus of claim 1, wherein the carrier frequency is between 1 and20 MHz.
 9. The apparatus of claim 1, wherein the movement of the surfacecomprises a vibration.
 10. A sensing apparatus comprising: a transducerhaving an ultrasound reflecting surface that moves in response to anapplied driving signal, wherein variations in the applied driving signalcauses corresponding variations in the velocity of the surface; a sensorthat generates an output signal that depends on a sensed parameter; anda control circuit that generates the driving signal that is applied tothe transducer based on the output signal generated by the sensor, suchthat variations in the value of the output signal result incorresponding variations in the driving signal, wherein the transducer'sresponse to an applied driving signal and the control circuit's responseto the output signal are repeatable such that the value of the outputsignal can be determined by measuring the variations in the velocity ofthe surface using externally applied Doppler ultrasound.
 11. The sensingapparatus of claim 10, wherein the transducer comprises a piezoelectricelement.
 12. The sensing apparatus of claim 10, further comprising abiocompatible housing, wherein the transducer, the sensor, and thecontrol circuit are all housed in the housing.
 13. The sensing apparatusof claim 11, further comprising a battery that provides power to thesensor and the control circuit, and wherein the battery is housed in thehousing.
 14. The sensing apparatus of claim 10, wherein the controlcircuit comprises a processor and a driver circuit.
 15. The sensingapparatus of claim 10, wherein the driving signal has a frequencybetween 50 and 1000 Hz.
 16. The sending apparatus of claim 10, whereinthe movement of the surface comprises a vibration.
 17. A method forcommunicating with an implanted sensor, the method comprising the stepsof: obtaining an output signal from a sensor, wherein the output signaldepends on a sensed parameter; generating, based on the output signalobtained from the sensor, a driving signal for driving a transducer,such that variations in the value of the output signal result incorresponding variations in the driving signal, wherein the drivingsignal is configured to cause an ultrasound reflecting surface of thetransducer to move, and wherein variations in the applied driving signalcauses corresponding variations in the velocity of the surface; applyingthe driving signal to the transducer, wherein the transducer's responseto the applied driving signal and the generation of the driving signalbased on the output signal obtained from the sensor are repeatable suchthat the value of the output signal can be determined by measuring thevariations in the velocity of the surface using externally appliedDoppler ultrasound; directing ultrasound energy at a carrier frequencyonto the transducer; detecting ultrasound reflections from the surfacethat have been shifted in frequency by the moving surface; using Dopplerprocessing to determine the velocity of the surface based on thedetected reflections; and mapping the determined velocity onto a valueof the sensed parameter.
 18. The method of claim 17, wherein the drivingsignal is selected to cause FM modulation of the carrier when thesurface moves.
 19. The method of claim 17, wherein the driving signalcomprises a triangular waveform.
 20. The method of claim 17, wherein thedriving signal has a frequency between 50 and 1000 Hz.
 21. The method ofclaim 17, wherein the carrier frequency is between 1 and 20 MHz.
 22. Themethod of claim 17, wherein the movement of the surface comprises avibration.